Hi Greg,
I 've downloaded and built RestCGI in Mac OSX El Capitán, however I found 2 errors while doing "make check", which I fixed.
The details are below:

$ make check
Making check in src
/bin/sh ../libtool --tag=CXX --mode=compile g++ -DHAVE_CONFIG_H -I. -I.. -g -O2 -MT env.lo -MD -MP -MF .deps/env.Tpo -c -o env.lo env.cpp
libtool: compile: g++ -DHAVE_CONFIG_H -I. -I.. -g -O2 -MT env.lo -MD -MP -MF .deps/env.Tpo -c env.cpp -fno-common -DPIC -o .libs/env.o
env.cpp:117:18: error: use of undeclared identifier 'environ'
p_ = environ;
^
1 error generated.
make[1]: *** [env.lo] Error 1
make: *** [check-recursive] Error 1

SOLUTION:
I added the following code below line 28 in file "src/env.cpp"

#else
extern char **environ;

Then I ran "make check" again and I got:

$ make check
Making check in src
/bin/sh ../libtool --tag=CXX --mode=compile g++ -DHAVE_CONFIG_H -I. -I.. -g -O2 -MT env.lo -MD -MP -MF .deps/env.Tpo -c -o env.lo env.cpp
libtool: compile: g++ -DHAVE_CONFIG_H -I. -I.. -g -O2 -MT env.lo -MD -MP -MF .deps/env.Tpo -c env.cpp -fno-common -DPIC -o .libs/env.o
libtool: compile: g++ -DHAVE_CONFIG_H -I. -I.. -g -O2 -MT env.lo -MD -MP -MF .deps/env.Tpo -c env.cpp -o env.o >/dev/null 2>&1
mv -f .deps/env.Tpo .deps/env.Plo
/bin/sh ../libtool --tag=CXX --mode=link g++ -g -O2 -version-info 0:0:0 -o librestcgi.la -rpath /usr/local/lib content.lo cookie.lo ctmpl.lo date_time.lo endpoint.lo env.lo exception.lo hdr.lo httpsyn.lo method.lo method_e.lo resource.lo rest.lo status_code_e.lo utils.lo version.lo -luripp
libtool: link: rm -fr .libs/librestcgi.0.dylib .libs/librestcgi.0.dylib.dSYM .libs/librestcgi.a .libs/librestcgi.dylib .libs/librestcgi.la .libs/librestcgi.lai
libtool: link: g++ -dynamiclib -Wl,-undefined -Wl,dynamic_lookup -o .libs/librestcgi.0.dylib .libs/content.o .libs/cookie.o .libs/ctmpl.o .libs/date_time.o .libs/endpoint.o .libs/env.o .libs/exception.o .libs/hdr.o .libs/httpsyn.o .libs/method.o .libs/method_e.o .libs/resource.o .libs/rest.o .libs/status_code_e.o .libs/utils.o .libs/version.o /usr/local/lib/liburipp.dylib -install_name /usr/local/lib/librestcgi.0.dylib -compatibility_version 1 -current_version 1.0 -Wl,-single_module
libtool: link: dsymutil .libs/librestcgi.0.dylib || :
libtool: link: (cd ".libs" && rm -f "librestcgi.dylib" && ln -s "librestcgi.0.dylib" "librestcgi.dylib")
libtool: link: ar cru .libs/librestcgi.a content.o cookie.o ctmpl.o date_time.o endpoint.o env.o exception.o hdr.o httpsyn.o method.o method_e.o resource.o rest.o status_code_e.o utils.o version.o
libtool: link: ranlib .libs/librestcgi.a
libtool: link: ( cd ".libs" && rm -f "librestcgi.la" && ln -s "../librestcgi.la" "librestcgi.la" )
Making check in test
\/Applications\/apps\/Xcode.app\/Contents\/Developer\/usr\/bin\/make main
g++ -DHAVE_CONFIG_H -I. -I.. -g -O2 -MT restcgi_resource.o -MD -MP -MF .deps/restcgi_resource.Tpo -c -o restcgi_resource.o restcgi_resource.cpp
restcgi_resource.cpp:194:9: error: reference to 'shared_ptr' is ambiguous
shared_ptr<test11> tr = dynamic_pointer_cast<test11>(rp.resource());
^
\/Applications\/apps\/Xcode.app\/Contents\/Developer\/Toolchains\/XcodeDefault.xctoolchain\/usr\/bin\/..\/include\/c++\/v1\/memory:3847:29: note:
candidate found by name lookup is 'std::__1::shared_ptr'
class LIBCPP_TYPE_VIS_ONLY shared_ptr
^
/usr/local/include/boost/smart_ptr/shared_ptr.hpp:762:36: note: candidate found by name lookup is
'boost::shared_ptr'
template<class Y=""> friend class shared_ptr;
^
restcgi_resource.cpp:195:21: error: use of undeclared identifier 'tr'
TEST_ASSERT(tr && tr->uri_path
.encoding() == "bar");
^
restcgi_resource.cpp:195:27: error: use of undeclared identifier 'tr'
TEST_ASSERT(tr && tr->uri_path_.encoding() == "bar");
^
3 errors generated.
make[2]: *** [restcgi_resource.o] Error 1
make[1]: *** [check-am] Error 2
make: *** [check-recursive] Error 1

SOLUTION:
I replaced line 194 in file "test/restcgi_resource.cpp" by

    boost::shared_ptr<test11> tr = dynamic_pointer_cast<test11>(rp.resource());

The process finished OK then.
I hope that this helps!

Hugo